Monofilaments are prepared from many different thermoplastic polymers for a variety of industrial and professional applications. The physical properties of such monofilaments, such as tensile strength, elongation and modulus, depend on the particular polymer composition and on the method of manufacture. It is known, for example, that melt spun nylon and polyethylene terephthalate polymers produce monofilaments of higher tenacity than, for example, the polyolefins, and that the tenacity of such monofilaments is increased while elongation is decreased by drawing the filaments under conditions to increase molecular orientation.
The use of thermoplastic monofilaments as surgical sutures is well established. Monofilament sutures formed of isotactic polypropylene are described in U.S. Pat. No. 3,630,205, and monofilament sutures of poly-dioxanone are described in U.S. Pat. No. 4,052,988.
Copolymers of p-dioxanone and glycolide useful in preparing monofilament sutures are described in U.S. Pat. No. 4,635,497, while sutures comprising poly [tetramethylene terephthalate-co-(alkenyl or alkyl) succinate] are described in U.S. Pat. No. 4,388,426. Other polymers suggested for use as synthetic absorbable sutures are disclosed in the literature and are well-known to those skilled in the art.
It is generally desirable for surgical sutures to Posses high tenacity (greater than 50,000 psi), low to moderate elongation (from 20-60%), and low modulus (less than 500,000 psi). Low modulus values signify a high degree of filament flexibility and limpness as opposed to a stiff, wiry material, a particularly desirable and even essential feature of a surgical suture. U.S. Pat. No. 3,630,205 describes a process whereby the flexibility of polypropylene sutures may be improved with little sacrifice in tensile strength by stretching the monofilament about 6.6.times. under controlled conditions and then relaxing to 76-91% of the stretched length.
U.S. Pat. No. 4,246,904 describes a surgical suture prepared from a segmented polyether-ester block copolymer which is reported to have excellent strength and flexibility compared to prior art monofilament sutures. The sutures of poly(p-dioxane) described in U.S. Pat. No. 4,052,988 are also characterized as possessing good tensile and knot strength and a high level of flexibility and softness.
The desirability of suture flexibility and softness as indicated by low modulus values is well recognized by the surgical profession and is a physical property constantly sought after in connection with the development of new surgical suture products. Some suture materials, such polylactide, polyglycolide, and copolymers of lactide and glycolide are generally considered to be too stiff and wiry to be used as monofilaments in all but the smallest suture sizes, and have found commercial acceptance in the larger sizes only as braided sutures, which are more flexible by virtue of their physical construction.
It is accordingly an object of the present invention to provide a process whereby monofilament sutures having improved softness and flexibility may be prepared. It is a further object of this invention to provide a process for reducing the modulus of existing monofilament suture materials with little or no loss of tensile strength. It is yet a further object of this invention to provide improved surgical monofilament sutures of poly(p-dioxanone) and other suitable polymeric materials characterized by reduced values of Young's modulus. A further object of this invention is to provide a method whereby the surface characteristics of thermoplastic suture monofilaments may be modified to enhance the properties of such suture monofilaments, particularly surface smoothness and integrity. These and other objects will be apparent from the ensuing description and claims.